We wish to define the entire domain of the bacteriophage T4 rIIB mRNA that is scanned by the ribosome during the initiation of protein synthesis. We will use exhaustive genetics to select mutations that diminish translation of the rIIB mRNA and biochemistry to determine contacts between ribosomes and the mRNA. We will ask the congruence between the genetic and biochemical answers. Additionally, we will study a translational repressor that interacts with the rIIB mRNA. We will use genetics and biochemistry here as well. Our goal is to understand in molecular terms a set of interactions that control gene expression in prokaryotes at the post-transcriptional level.